The objective of wind energy is to generate electricity from the wind through wind turbines with the maximum possible efficiency and the minimum cost. The most spread configurations of wind turbines imply the use of one tower for the installation of the rotor at some predetermined elevation relative to the ground.
These towers are designed to withstand the set of reactions generated as a consequence of the loads to which both the wind turbine's rotor and the nacelle are subjected. The ideal position of the rotor to prevent the ground effect together with the blade diameters cause the optimal height at which it should be placed to increase as the wind turbine's electric power increases. In these machines with large power it is common to use towers with a height greater than 100 meters.
The attachment area, where the tower is attached to the foundation is one of the critical design points of the wind turbine and it has an impact in the latter's structural integrity.
The manufacturing of attachments between the tower's base and the foundation is known in the state of the art, said manufacturing essentially comprising the following steps:
First, a foundation hole is made in which a set of bolts, over which the first the first section of the tower is to be connected, are installed. Subsequently, concrete is poured in the hole in a way that the bolts protrude thereof.
Later, a first section of the tower is added over the already set concrete, wherein said concrete will be in contact with levelling metal sheets placed between the aforementioned first section and the foundation's concrete, said sheets intended for correcting the manufacturing dimensional tolerances of the foundation and correctly positioning the first section of the tower.
Once the first section of the tower has been levelled over the foundation, a cylindrical ring is installed on the perimeter surrounding the section and grout is poured over the ring. The grout collaborates in the attachment of the tower to the foundation establishing an adhesive connection between the grout and the tower on the upper section of the grout and between the grout and the foundation on the lower section of the grout.
Finally, once the grout has set, torque is applied to the bolts for tightening said bolts, the attachment being thereby finished.
Other methods for the attachment of the tower to the foundation are also known, in which a previous stage of making the hole is also executed which is then filled with concrete and from which the aforementioned bolts for connection to the tower's base also protrude. Subsequently, a support element is located over the foundation for the transmission of stresses from the tower to the foundation which may be, for example, made of steel or concrete, and which is placed over the concrete poured in the foundation hole once said concrete has set.
Subsequently, the levelling of the support element is carried out and then filling grout is poured in the space located between the support element and the foundation concrete, for the connection of the support element to the foundation. Once the filling grout has set, the upper side of the support element is connected to the tower's base through connection bolts which go through the aforementioned support element. In this case, an adhesive connection is established between the grout and the support element on the grout's upper side and between the grout and the foundation on the grout's lower side.
In both cases, the use of grout as an intermediate element and the reaction stresses generated at the tower's base determine, for a design of the predetermined tower section, the choice of the type of foundation for a specific location, since the strength mechanical capacity of the grout is limited and, at each location, the stresses vary depending on the winds associated to each aforementioned location. This creates the need to vary either the dimensions of the hole or its strength capacity for each type of tower.
Other disadvantages are the additional setting time that is implied by the use of grout, which increases the tower's construction time, as well as the influence of climatological conditions on the grout's setting time and also on the quality of the grout itself and, therefore, on the quality of the connection or even on the impossibility to be able to pour the grout.
Finally, another disadvantage of the use of grout are the associated quality problems, since the quality of the grout's seal is critical and difficult to control and even to unify for each wind turbine, since it depends, in addition to the climatological conditions previously discussed, on the experience of the workers or the existence of bubbles inside the grout which weaken the mix and which may appear as a consequence of a variation of the direction and speed of the grout in each area throughout the surface with regards to previous calculations, these parameters being difficult to control as they are affected by numerous variables.
In the first attachment method of a tower to the foundation known in the state of the art and described above, there is an additional given disadvantage: since the stage of levelling is applied to the tower's first section, it is necessary to use a high tonnage crane to move the first section during the levelling stage, thereby incurring in additional installation costs and the possibility of a damage to the components.